Photographic silver halide emulsion



Patented Feb. 16, 1943 PHOTOGRAPHIO SILVER HALIDE EMULSION Wesley G.Lowe, Rochester, N. Y., aslignor to Eastman Kodak Company, Rochester, N.Y., a corporation of New Jersey No Drawing. Application October 29,1941, Serizi 2 Claims.

*This invention relates to photographic silver halide emulsion in whichpolyvinyl alcohol is employed as the protective colloid for the silverhalide therein. This invention also relates to photographic silverhalide emulsions in which a polyvinyl alcohol, gelled by means ofa-naphthol, is employed as the protective colloid.

This is a continuation-in-part application of my U. S. applicationSerial No. 318,559, filed February 12, 1940, now Patent No. 2,286,215.

Gelatin has ordinarily been employed as the protective colloid for thelight-sensitive silver halides in photographic emulsions due to itsability to hold these salts in suspension, its good water permeability,its insolubility in water and in photographic developing solutions atordinary tempertaures and its ability to form a thin coating of thelight-sensitive silver halide emulsion upon the film base. i

Various other materials have been suggested for use as the protectivecolloids in photographic emulsions but in most instances the suggestedmaterials have evidenced inferiority in one or more of the propertiesenumerated, compared with gelatin. For instance, cellulose esters, suchas cellulose nitrate, have been suggested as carriers for silver saltsin photographic emulsions. The emulsion layer formed, however, when dry,is none too permeable to developing solutions and, therefore, cellulosenitrate has not been successful with theconventional developing andfixing baths. Another disadvantage of cellulose nitrate is itsinsolubility in solvent mixtures containing appreciable amounts ofwater. This prevents the use of the water-soluble salts which are nowordinarily employed in forming lightsensitive photographic emulsions.

Various other colloidal materials have been mentioned in thisconnection. However, most of those that have been suggested heretoforeare deficient for some reason or other, such as lack of good waterpermeability, inability to hold silver halide in suspension or inabilityto form a hard emulsion layer when coated upon a support, such asphotographic film base or paper, and dried.

Gelatin as a protective colloid for photographic silver halide emulsionsleaves much to be desired. Gelatin is susceptible to depredations byorganisms, both animal and vegetable. For instance, insects attackgelatin, particularly in the tropics. Gelatin is susceptible to theaction of molds and of bacteria, particularly under conditions of highhumidity and a. compatible temperature. Gelatin, because of its naturalsource often varies in properties from time to time.

In Great Britain February 1 Also, to obtain gelatin in a highly refinedcon dition (as needed for photographic purposes) requires considerablecare and it can be procured in limited amounts only from animal productsby a slow and expensive process.

One object of my invention is to provide carriers or protective colloidsfor light-sensitive silver halides, which can be employed instead ofgelatin forvarious photographic silver halide emulsions. Another objectof my invention is to provide carriers orprotective colloidsforlightsensitive silver halides in photographic emulsions which possessmany of the advantages but which lack some of the disadvantagesevidenced by gelatin. Other objects will appear herein. 1 have foundthat polyvinyl alcohol is. eminently suitable as a carrierior the silverhalide in photographic emulsions, the physical properties of the-material being easily controlled in production so as to get a materialof constant properties. Polyvinyl alcohol has the ability to hold silverhalide in suspension, is easily. coated out upon a support, such aspaper, readily sets and dries to a hard, scratch-resistant emulsionlayer, which may be treated with developing and fixing baths, and it may.be washed thoroughly with water after processing without dissolving. Ihave found that a polyvinyl alcohol emulsionlayer swells but does notdissolve when developed, fixed and washed in the usual way. Afterdrying, it exhibits good physical properties. I have found thatphotographic silver halide emulsions having polyvinyl alcohol as theprotective colloid, in which an amount of a-naphthol has beenincorporated therewith to cause gelling of the polyvinyl alcohol, havegood photographic properties. The treatment of an aqueous solution ofpolyvinyl alcohol with u-naphthol to convert the polyvinyl alcohol to arigid gel, which is thermalreversible, is described and claimed in U. S.Patent No. 2,249,536 of McDowell and Kenyon. This is distinct from thehardening action of some materials on polyvinyl alcohol.

A photographic silver halide emulsion in accordance with my inventionmay, for instance, be prepared after a silver halide has been dispersedin aqueous polyvinyl alcohol, such as 540% concentration, by treatingthe mass. with an amount of a-naphthol at a slightly elevatedtemperature (preferably 40-90 C.) which, when cooled, forms a rigidthermal-reversible gel. After the cooling has taken place, the emulsiongel is formed and may be washed or otherwise treated as desired. Thisphenomena is valuable in that it makes possible the coating of theemulsion upon a support while the mass is liquid at the elevatedtemperature and then gel-setting of the emulsion. The emulsion can thenbe dried and forms a photo-sensitive layer on the support. The gellingagent should only be added after the formation of the emulsion tosatisfactorily form a gelled product.

"Polyvinyl alcohols, which term as used herein includes alsowater-soluble hydrolyzed polyvinyl esters having a polyvinyl estercontent of approximately 5% or sometimes less-the remainder beingpolyvinyl alcohol-are particularly adapted to the preparation ofunwashed emulsions and when coated on a suitable support such as paperin the form of a photosensitive silver halide emulsion will recordimages accurately and sharply, forming permanent prints when processedin the ordinary manner.

Polyvinyl alcohols are eminently suitable for the preparation of washedphotographic emulsions in accordance with my invention. These emulsionsare particularly suitable for coating onto a cellulose ester support andwhen so coated in the form of a photosensitive silver halide emulsionwill, upon proper exposure, developing and printing, give a sharp image.

The photographic emulsions of my invention may be prepared by a processinvolving first dispersing silver halide in polyvinyl alcohol, such asby running aqueous solutions of silver nitrate and alkali metal halideinto an aqueous solution of polyvinyl alcohol. Preferably after adding agelling amount of a-naphthol and warming to cause better dispersion andincrease the speed, as is well known in the art, followed by coolingand, ifv desired, sensitizing with a small amount of a sensitizing dye,theemulsion is ready to coat out onto a support, such a paper. Theemulsion is coated out in a warm condition so as to render it fiowable.After the emulsion layer is dried, such as by a current of warm air, itmay be exposed, processed, washed and dried in the usual manner andprints of good appearance result. Where the a-naphthol gelling agent isused, the emulsion, after coating out onto a support, can be set bycooling.

It is preferable to incorporate the gelling agent in the emulsioncomposition after the formation of the silver halide because otherwisethere may sometimes be a tendency towards fog. The gelling agent may beadded after the addition of the polyvinyl alcohol protective colloid, orit may be incorporated in polyvinyl alcohol which is mixed with thesilver halide. In the case of the washed emulsions it is often desirableto add some m-naphthol thereto after the washing operation to replaceany gelling agent which was lost in the washing operation, prior tocoating of the emulsion on to the support.

a-naphthol or other like gelling agents convert polyvinyl alcohol to athermal-reversible gel as described in McDowell and Kenyon Patent No.

1 2,249,536. This gelled polyvinyl alcohol forms a graphic emulsions inaccordance with my invention need be only a gelling amount. For instancethe usual emulsions using p lyvinyl alcohol as the protective colloid asfinally prepared, contain about 3-5% of polyvinyl alcohol. For 50 gms.of emulsion usually approximately V, or a gm. of a-naphthol will besatisfactory altho the amount of a-naphthol within the limits given is amatter of choice. ,In preparing a washed emulsion some of thegellingagent is usually washed out and therefore may be employed in a somewhatgreater amount. However, after the washing operation it is desirablethat the amount of a-naphthol be not so great as to result in theformation of crystals in the emulsion or that will render the handlingof the emulsion more difllcult than necessary because of excessivetoughness or a higher melting point or both.

a-naphthol is distinctive as a gelling agent for polyvinyl alcoholphotographic emulsions in that the emulsions which have been gelled witha-naphthol exhibit an adherence between the emulsion layer and acellulose nitrate subbing layer, which is superior, as a rule, than isthe case with other emulsions in which a gelled polyvinyl alcohol is theprotective colloid.

My invention is concerned with the preparation of photographic silverhalide emulsions adapted to be coated upon paper or upon a transparentbase, such as cellulose derivative film base or glass plates, to formwhat is normally known in the art as photographic paper, photographicfilm or photographic plates.

The photographic silver halide emulsions of my invention may besensitized in the same manner as are silver halide emulsions generally.This is ordinarily accomplished with my emulsions by thoroughlyincorporating a small amount of a sensitizing dye after the emulsionprepared. If staining of the paper on which the emulsion is coated isobjectionable, an amount of sensitizing dye, which is not great enoughto cause staining, may be employed. If a more sensitive emulsion isdesired and staining is not objectionable, or 'a support which will-notabsorb the dye is used, a larger proportion of sensitizing dye may beincorporated. The emulsions, containing the smaller amounts ofsensitizing dye, are ordinarily adapted for making contact prints orenlargements by projection.

The sensitizers, which have been previously employed to opticallysensitize silver halide emulsions, may be incorporated in the emulsionsof my invention to increase their sensitivity. Examples of sensitizers,which may be employed in the emulsions-described herein, are thecyanines and the sensitizing dyes disclosed in the following patents:

' U. S. No, 2,186,608, Keyes, January 9, 1940; U. S.

' No. 2,165,339, Brooker, July 11, 1939.

The following examples illustrate my invention:

Example I cc. of a 10% solution of polyvinyl alcohol I was diluted withcc. of water. Into this the following solutions were run with vigorousstirring, the precipitation time being five minutes. Solution A: 40 cc.of a 50% solution of silver nitrate, Solution B: 12.8 grams of potassiumbromide, 3 grams of potassium chloride, 0.5 gram of potassium iodide and44 cc. of water. 80 cc. of a 10% solution of polyvinyl alcohol was thenadded :and the emulsion was stirred for 20 minutes at 50. C. 120 cc. ofa solution of polyvinyl a1- cohol was added and the emulsion was cooled.50grams of this emulsion was heated to 40 C. and Amie. gram ofa-naphthol was added. One milligram of2,3-diethyl-4'-methyl-oxathiazolocarbocyanine dissolved ina littlemethyl alcohol was added-tosensitize the emulsion. The emulsion wascoated on a glass plate and found to gel in 2-3 minutes at roomtemperature and more rapidly when chilled. The plate was exposed wet andprocessed. It was found to develop and fix readily. Y

A plate coated as before was washed by placing in cold water for 30minutes and was then dried.

On exposing and processing, the plate was found to develop and fixreadily giving a clear, clean result. The emulsion was alsocoated onpaper. It was dried by subjecting to a current of warm air. Uponexposing and processingdt' was found to give a clear,'clean result.

Example II bromide and 0.25 gram 'of potassium iodide made loweredtemperature.

up to 21cc. with water. grams of 10% polyvinyl alcohol solution was thenadded followed by the addition of 10 cc. of glacial acetic acid. Theemulsion was then heated to 145 F. and 20 cc. of a 6% solution ofa-naphthol in ethanol was added. The emulsion was boiled for 16 hours.It set to a flrm gel which was cut into strips and thoroughly washedwith water. The washed emulsion was melted by heating to F. and

10 cc. of a 6% solution of a-naphthol was added thereto. The emulsionwas coated on glass plates. It set to a gel and dried rapidly. 'Theseplates were exposed and processed. They were found to be of medium speedand to give a clear. clean result. 1

1. A light-sensitive photographic emulsion comprising a silver halidedispersed in a thermalreversible gel composed of polyvinyl alcohol mixedwith a gelling amount of a-naphthol.

2. A method of preparing a light-sensitive photographic emulsion whichcomprises preparing a silver halide in an aqueous'solution of apolyvinyl alcohol and subsequently adding thereto at a temperature of40-90" C. a sufflcient amount of a-naphthol to form a rigid gel at aWESLEY G. LOWE.

